Centrifugal compressor



March 1968 L -c. F. KOENIG m 3,372,352

' CENTRIFUGAL COMPRESSOR Filed Oct. 22, 1965 4 Sheets Sheet 2 I PERCENTOF DESIGN HEAD INLET VOLUME FLOW INVENTOR G CARL F. KOEN|G-,]]I

BY V

ATTORNEYS March 12, 1968 c. F. KOENIG m I CENTRIFUGAL COMPRESSOR 4Sheets-Sheet 4 Filed on. 22, 1965 CONTROLLER 5 INVE NTOR CARL F.KOENIGIE ATTORNEYS United States Fatent O 3,372,862 CENTRIFUGALCOMPRESSOR Carl F. Koenig III, Montgomery, Ghio, assignor to De LavalTurbine Inc., Trenton, N.J., a corporation of Delaware Filed Oct. 22,1965, Ser. No. 501,865 2 Claims. (Cl. 230114') ABSTRACT OF THEDISCLGSURE A centrifugal compressor is provided with fixed and movablediffuser vanes disposed around the periphery of its impeller. Themovable diffuser vanes are in the form of airfoils pivoted and free torotate about axes in order to provide variable throat areas between theleading and trailing edges of adjacent vanes. The vanes are pivoted sothat this throat area is directly proportional to flow rate andinversely proportional to absolute discharge velocity so that optimumpressure and efiiciency are obtained. The movable vanes may be providedwith dampers, and may be connected to control an outlet by-pass valve inorder to prevent the occurrence of a surge condition.

This invention relates to centrifugal compressors, and particularly tothose having movable diffuser vanes.

In compressors having constant impeller speeds, the performancecharacteristics for a given flow rate can be improved considerably bythe vaned diffusers. For a given flow rate, both the efficiency and thedischarge pressure can be increased by the use of vaned diffusers.Compressors with vanelcss diffusers, of course, are capable of operatingover a wider range of flow rates with a more or less constant efficiencyand a more or les flat pressure-flow characteristic over a wide range offlow rates. Compressors having fixed vane diffusers, on the other hand,can operate with high efficiency and against a higher head, but therange of flow rates which they are capable of handling is severelylimited since they have a high surge How and a very low maximum flow.

The efiiciency-flow and the pressure-flow characteristics of acentrifugal compressor can be varied by providing adjustable diffuservanes, and the efficiency and pressure can be optimized for a given flowrate.

It is the general object of this invention to provide a centrifugalcompressor having adjustable vanes, in which the adjustment occursautomatically as flow conditions change.

In centrifugal compressors, where losses in the impeller and in thediffuser become high enough so that the pressure varies directly ratherthan inversely with the flow rate, a surge condition will occur. Beforethis condition occurs, it is desirable to by-pass the output of thecompressor to direct it away from the process being supplied by thecompressor. The movable vanes which accomplish automatic adjustment ofthe characteristics of the compressor are also capable of detecting theonset of a surge condition. Accordingly, a further object of theinvention is to provide a control whereby the output of a compressor isdirected away from the process which is supplied by the compressor forthe duration of a surge condition.

A still further object of the invention is to provide positive meanswhereby the delivery of a centrifugal compressor can be adjusted.

Other objects will be apparent from the following description read inconjunction with the accompanying drawings in which:

FIGURE 1 is a section of a centrifugal compressor 3,372,862 PatentedMar. 12, 1968 stage showing a movable Wane in accordance with theinvention;

FIGURE 2 is a partially diagrammatic section taken on the plane 22 ofFIGURE 1;

FIGURE 3 is a sectional view of a damping mechanism in accordance withthe invention;

FIGURE 4 is a diagram showing efficiency vs. flow curves for acompressor employing a vaneless diffuser and for a compressor employingan adjustable vaned diffuser in various positions of adjustment;

FIGURE 5 is a diagram illustrating pressure-flow characteristics for acompressor employing a vaneless diffuser and for a compressor employingan adjustable vaned diffuser in positions of adjustment corresponding tothe diffuser positions from which FIGURE 4 was obtained;

FIGURE 6 is a vector diagram illustrating the opera tion of theautomatically adjust-able diffuser vanes in accordance with theinvention;

FIGURE 7 is a diagram of a control system for redirecting the output ofa compressor in the event of a surge condition;

FIGURE 8 is a diagram illustrating the variation of diffuser vane anglewith flow rate and the variation of outlet pressure with inlet volumeflow of a compressor equipped with the control system illustrated inFIG- URE 7;

FIGURE 9 is a section of a centrifugal compressor stage provided with apositive means for adjusting the position of a diffuser vane inaccordance with the invention;

FIGURE 10 is a sectional view of the positive diffuser vane adjustingmeans taken on the plane 10-1t) of FIG- URE 9;

FIGURE 11 is a diagram showing the stage headvolume characteristic for atypical three-stage compressor which is not provided with theautomatically adjustable diffuser vanes in accordance with theinvention; and

FIGURE 12 is a partially diagrammatic illustration of a centrifugalcompressor and a system for redirecting its output in the event of asurge condition.

Referring to FIGURE 1, a driving shaft 12 is shown on which is mountedin a suitable manner a radial impeller 14. A passage 16 is formedbetween housing members 18 and 20, and within this passage there isprovided a plurality of stationary diffuser vanes 22 spaced from oneanother uniformly around the circumference of the com pressor stage.

Adjacent the leading edge of each of the fixed diffuser vanes 22, thereis provided a movable diffuser vane 24 fixed to a pair of cylindricalmembers 26 mounted to rotate in bearings provided in housing members 18and 29 so that vanes 24 can rotate freely about their respective axesparallel to the axis of rotation of drive shaft 12. Vanes 24 are in theform of highly cambered airfoils, and are pivoted so that the axes aboutwhich they rotate pass through them near and parallel to their leadingedges. The distance between the leading edge and the axis of rotation isdesirably from 20 to 25% of the chord length of the airfoil. The axis ofrotation of each of the airfoils 24 passes through the centers of itscorresponding member 26. The pivot position with respect the airfoil isa design parameter and determines the airfoil position as a function ofthe flow rate.

*Each movable diffuser vane 24 is fixed to a shaft 28 which passesthrough a bearing 30 in housing member 20. Referring to FIGURES 1 and 3,shaft 28 is suitably keyed to a crank arm 32, which is pivoted at itsother end to a shaft 34 driving a piston 36 within 'a dashpot 38.Dashpot 38 is desirably oil-filled, and a restricted orifice 40 inpiston 36 provides a strong damping force to resist rapid movement ofthe diffuser vanes 24.

In this embodiment of the invention the dashpot 33 is provided for eachmovable van 24, and each dashpot is mounted by means of a pin 42 fixedto housing member 20 and passing through a hole 44 provided at the topof each dashpot. Suitable seals 46 are provided at the opening throughwhich piston rod 34 passes. The attitude of the vanes 24 is affected bythe direction of flow of the stream entering. the diffuser, which, inturn, is affected by the quantity of flow. Vanes 24 tend to line up withthe direction of the stream entering the diffuser, and thus the attitudeis dependent on the quantity of flow. The degree of damping and the rateof movement of an airfoil for a given torque can be adjusted by alteringthe size of the orifice 4-9 in the associated dashpot.

The relationship between the position of the movable vanes and the flowwill best be understood from reference to FIGURES 2 and 6. c is an anglerepresenting the direction of flow. U represents the peripheral speed ofthe impeller and is constant as long as the impeller speed is unchanged.W is the relative velocity vector and varies in magnitude directly asthe rate of flow, but remains at essentially a constant angle 5 with theperipheral component U. C is the absolute discharge velocity.

As the rate of flow decreases, W decreases to W, at

decreases to a and C, the absolute velocity, changes both in magnitudeand direction to C. To obtain optimum performance, the diffuser throatarea should now be decreased by the ratio Q'C/ QC where Q is theoriginal flow rate, and Q is the new flow rate. The throat area of thediffuser is represented by the diameter X of the circle interposedbetween the trailing edge of one vane 24 and the leading edge of anadjacent vane.

The pivot axes of the vanes 24 are located so that the change in thediffuser throat area varies according to the above relationship. Theposition of the pivot axis which accomplishes this relationship isordinarily at a distance between to 25% of the chord length of theairfoil from its leading edge. It will be apparent that, as flowincreases, the diffuser throat area increases, and as flow decreases,the area decreases.

Reference should now be made to FIGURE 4 which shows typical curvesrepresenting efiiciency vs. flow. Curve E represents the characteristicof a typical vaneless diffuser. Each of curves A, B, C and D representsthe characteristic of a compressor having a vaned diffuser in which thevanes are in various positions of adjustment. It will be apparent thatthe peak of each efficiency vs. flow curve is in a different position,and that if the characteristics of a compressor were made continuouslyvariable according to the flow, the compressor can be made to operatenear maximum efficiency throughout a wide range of flow rates. Themaximum efliciency is considerably above that obtainable from acompressor having a vaneless diffuser.

The automatic adjustment of the diffuser throat area described abovechanges the characteristic efficiency vs. flow curve in accordance withthe flow rate so that, at high rates of flow, the characteristic curveis, for example, the position of curve A corresponding to high rate offlow since the diffuser throat area is relatively large. At low rates offlow, the diffuser throat area automatically becomes small, andconsequently the effective efficiency curve is, for example, curve B.The resulting effective efficiency curve is, therefore, the envelope ofthe individual curves.

FIGURE 5 shows a pressure-flow characteristic F for a typical compressorhaving a vaneless diffuser for the purpose of comparison, and curves G,H, I and I which represent the pressure-flow characteristicscorresponding to different adjustable vane positions. The position ofthe vane represented by curve G corresponds to the vane positionrepresented by curve A in FIGURE 4, H corresponds to B, I toC and I toD. By the present invention, the compressor is made to operate on theenvelope of the pressure-flow characteristics. Pressure is thereforemade 4. practically independent of flow rate, the envelope having anapproximately fiat portion over a large range of fiow axis.

An off-design flow rate, the change in attitude of the movable vanes 24-will result in an opening between the trailing edges of the vane and theleading edges of the corresponding stationary diffusers. These openingsform effective boundary layer control slots. Flow separation isretarded, and the performance of the stationary diffuser passage atoff-design flows is greatly improved over that to be expected withconventional adjustable diffusers.

Since the movable vanes change their attitude with flow rates, they arecapable of detecting the onset of the surge condition in the compressorand of controlling operation to prevent surge as follows:

Referring to FIGURE 7, the symbolized compressor 4-8 is provided with aninlet 56 and an outlet 52 leading to a process. A controllable, normallyclosed bypass valve V is provided to direct the discharge of thecompressor away from the process for the duration of the surgecondition. A suitable valve controller 54 is provided with the signalobtained from and corresponding to the position of a vane 24 in thecompressor. The signal might, for example, be obtained from apotentiometer (not shown) driven by shaft 28 to which the movable vaneis attached. The controller 5 3 is desirably continuous in its operationso that the degree of opening of valve V corresponds to the attitude ofthe adjustable vane driving the controller 54. Such controllers arewell-known in the art and need not be described in detail.

In FIGURE 12, the connection of the controller and by-pass valve to thecompressor are shown. Compressor 48 is provided with a fixed diffuservane 22, and a movable vane 24 adapted to rotate shaft 28. Outletpassage 55 from the compressor is shown partly diagrammatically and line57 connects line 55 through valve 59 to an additional outlet line 61.Controller 54, which is of the wellknown servo type, is arranged to openvalve 59 proportionately in correspondence with the position of shaft 28so that, when vane 24- is in a position corresponding to the onset of asurge condition in the compressor, valve 59 is opened in order toprovide an additional outlet for gas from the compressor to counteractthe surge condition.

Referring to FIGURE 8, a typical pressure-flow characteristic is shown,and the surge point is indicated at Q The corresponding vane angle vs.flow curve is shown. The controller is designed so that valve V beginsto open when the flow reaches point K, and is fully opened when the Howreaches point L at the onset of the surge condition. The surge conditionoccurs at a flow rate Q and the vane angle at this time is oc (It willbe apparent that various suitable and known electric, hydraulic orpneumatic control devices can be used to control valve V in response tovariations in the position of the movable vane.)

It is often desirable to provide positive means within a compressor foradjusting the delivery independently of the process supplied by thedischarge of the compressor. An alternative arrangement illustrated inFIGURES 9 and 10 accomplishes this result.

A plurality of movable vanes 24 are fixed to rotatable shafts disposedcircumferentially around the periphery of the impeller. On each of theseshafts there is suitably fixed a pinion 58 in an annular space 60provided in the housing. A ring gear 62, also provided in space d0meshes with each of pinions 58 so that the vanes are no longer capableof moving independently. At least one of pinions 53 is keyed to acontrol shaft 64 rotatable within bearings 66 and 68 in the housing.Shaft 64 extends to the exterior of the housing for connection to anexternal actuator. Thus, by operation of the actuator, the throat areaof the diffusers can be varied so that any desired pressure-flowcharacteristic can be obtained. The actuator of shaft 64 may be made tooperate in response to signals corresponding to the process flow,process pressure or any other parameter to which the compressor deliveryis related. Alternatively, the position of shaft 64 can be adjustedmanually.

The invention, in the embodiment in which the movable diffuser vanes arefree to align themselves with the fluid stream, produces particularlyadvantageous results in multiple stage compressors wherein each of thestages is equipped with movable dilfuser vanes in the form of airfoils.The difficulties involved in ordinary multiple stage compressors resultfrom the fact that the stages can be perfectly matched to one another atonly one flow rate. These difiiculties will be apparent from FIGURE 11which shows the pressure-flow characteristics for each stage of atypical three-stage compressor with fixed diffusers. The abscissa is theinlet volume flow, and the ordinate represents percentage of the designhead. Where the operating point for stage one is A at the design inletvolume flow, the operating points of stages 2 and 3 are A and Arespectively. The stages are perfectly matched at these particularoperating points, since each stage is operating against its design head.If the inlet volume flow to the first stage is increased so that theoperating point is B because of the reduced compression in the firststage, the second stage operates at the point B so that the compressionin the second stage is somewhat below that in the first stage, and thecompression in the third stage is likewise below that in the secondstage. Each of the stages is operating at a different percentage of itsdesign head as is represented by the operating points 13,, B and B Asthe inlet volume flow to the first stage is further increased, theseparation of the operating points of the stages becomes greater, andthe mismatching of the stages increases as the inlet volume flowincreases.

The problem of mismatching at off design flow rates is avoided in amultiple stage compressor in which each of the stages is equipped withthe self-adjusting diffuser vanes of the invention, since each of thestages will have a broad operating range. Since the operating pressureis made less dependent on flow rate by the self-adjusting diffuservanes, the stages will remain matched over a broad range of flow rates.The overall performance of a multiple stage compressor is thus greatlyimproved and the provision of the self-adjusting vanes of the inventionis very desirable where inlet volume flow is likely to vary.

Another problem inherent in centrifugal compressors is caused by unevenflow distribution around the periphery of the impeller which results inunequal flow angles (a) in the fixed vane diffusers. The self-adjustingdiffusers of the invention adjust themselves to whatever flow angle (a)is required at any point on the periphery of the impeller. Losses due tounequal flow distribution are thus minimized.

The invention is further applicable to compressors comprising aplurality of stages operating in parallel. If the flow rate is not equalin all of the parallel stages, in ordinary compressors the overallefficiency is lowered and the overall operating range is reduced. Whereselfadjusting diffuser vanes are provided in each of the stages, eachstage adjusts itself to optimum performance,

and the overall performance is not affected by small differences in flowin the paralleled stages.

It will be apparent that the invention is not limited to use inconjunction with closed radial impellers and that it may be applied tocompressors having various types of impellers and various configurationsother than those disclosed. It will also be apparent that variousmodifications can be made to the invention without departing from itsscope as defined in the following claims.

What is claimed is:

1. In combination, a centrifugal compressor having inlet and outletpassages for each of its stages, a rotatable vane in the form of anairfoil disposed in at least one of said outlet passages and pivotednear its leading edge so that it aligns itself with the direction offlow of gas through said one of said outlet passages, control meansresponsive to the position of said rotatable vane and valve meansoperable by said control means and connected to provide an additionalpath for the discharge from said compressor when fiow of gas throughsaid one of said outlet passages is in the direction accompanying asurge condition.

2. A centrifugal compressor comprising at least one stage having inletand outlet passages, an impeller disposed within said stage for pumpinga gas introduced at said inlet passage through said outlet passage andproducing a substantial radial component of flow through said outletpassage, a plurality of vanes disposed within said outlet passagecircumferentially about said impeller means, said vanes being in theform of air-foils, the trailing portion of each said vane cooperatingwith the leading edge of the next adjacent vane to provide a variablethroat area for the escape of gas through said outlet p-assage, andmeans pivoting said vanes for free rotation about individual 'axessolely in response to the flow over said vanes, said pivoting meansbeing positioned with respect to said vanes so that the distance betweenthe leading edge of each of said vanes and its axis of rotation isbetween 20 and 25 percent of the chord length of the vane, whereby saidthroat area varies directly with discharge flow rate and inversely withabsolute discharge velocity.

References Cited UNITED STATES PATENTS 1,771,711 7/1930 Hahn 230-1142,189,252 2/1940 Reggio 230-114 2,382,913 8/1945 Robinson 230-1142,985,427 5/1961 Hou-ghton 230-114 3,014,430 12/1961 Schneider 103-973,237,565 3/1966 Hartland 103-97 3,268,155 8/1966 Hornschuch 230-115FOREIGN PATENTS 721,384 12/1931 France.

302,953 12/ 1928 Great Britain.

762,406 11/ 6 Great Britain.

861,630 2/ 1961 Great Britain.

HENRY F. RADUAZO, Primary Examiner. DONLEY J. STOCKING, Examiner.

